In borehole geophysics, a wide range of parametric borehole measurements can be made, including chemical and physical properties of the formation penetrated by the borehole, as well as properties of the borehole and material therein. Measurements are also made to determine the path of the borehole during drilling to steer the drilling operation or after drilling to plan details of the borehole. To measure parameters of interest as a function of depth within the borehole, a drill string can convey one or more logging-while-drilling (LWD) or measurement-while-drilling (MWD) sensors along the borehole so measurements can be made with the sensors while the borehole is being drilled.
As shown in FIG. 1A, a drill string 30 deploys in a borehole 12 from a drilling rig 20 and has a bottom hole assembly 40 disposed thereon. The rig 20 has draw works and other systems to control the drill string 30 as it advances and has pumps (not shown) that circulate drilling fluid or mud through the drill string 30. The bottom hole assembly 40 has an electronics section 50, a mud motor 60, and an instrument section 70. Drilling fluid flows from the drill string 30 and through the electronics section 50 to a rotor-stator element in the mud motor 60. Powered by the pumped fluid, the motor 60 imparts torque to the drill bit 34 to rotate the bit 34 and advance the borehole 12. The drilling fluid exits through the drill bit 34 and returns to the surface via the borehole annulus. The circulating drilling fluid removes drill bit cuttings from the borehole 12, controls pressure within the borehole 12, and cools the drill bit 34.
Surface equipment 22 having an uphole telemetry unit (not shown) can obtain sensor responses from one or more sensors in the assembly's instrument section 70. When combined with depth data, the sensor responses can form a log of one or more parameters of interest. Typically, the surface equipment 22 and electronics section 50 transfer data using telemetry systems known in the art, including mud pulse, acoustic, and electromagnetic systems.
Shown in more detail in FIG. 1B, the electronics section 50 couples to the drill string 30 with a connector 32. The electronic section 50 contains an electronics sonde 52 and allows for mud flow therethough. The sonde 52 includes a downhole telemetry unit 58, a power supply 54, and various sensors 56. Connectors 42/44 couple the mud motor 60 to the electronics section 50, and the connector 42 has a telemetry terminus that electrically connects to elements in the sonde 52.
Mud flows from the drill string 30, through the electronic section 50, through the connectors 42/44 and to the mud motor 50, which has a rotor 64 and a stator 62. The downhole flowing drilling fluid rotates the rotor 64 within the stator 62. In turn, the rotor 64 connects by a flex shaft 66 to a drive shaft 72 supported by bearings 68. The flex shaft 66 transmits power from the rotor 64 to the drive shaft 72.
Disposed below the mud motor 60, the instrument section 70 has one or more sensors 74 and electronics 76 to control the sensors 74. A power supply 78, such as a battery, can power the sensors 74 and electronics 76 if power is not supplied from sources above the mud motor 60. The drill bit (34; FIG. 1A) couples to a bit box 36, and the one or more sensors 74 are placed as near to the drill bit (34) as possible for better measurements. Sensor responses are transferred from the sensors 74 to the downhole telemetry unit 58 disposed above the mud motor 60. In turn, the sensor responses are telemetered uphole by the unit 58 to the surface, using mud pulse, electromagnetic, or acoustic telemetry.
Because the instrument section 70 is disposed in the bottom hole assembly 40 below the mud motor 60, the rotational nature of the mud motor 60 presents obstacles for connecting to the downhole sensors 74. As shown, the sensors 74 are hard wired to the electronics section 50 using conductors 46 disposed within the rotating elements of the mud motor 60. In particular, the conductors 46 connect to the sensor 74 and electronics 76 at a lower terminus 48a and extend up through the drive shaft 72, flex shaft 66, and rotor 64. Eventually, the conductors 46 terminate at an upper terminus 48b within the mud motor connector 44. As with the lower terminus, this upper terminus 48b rotates as do the conductors 46.
Running conductors 46 through the flex shaft 66 creates difficulties with sealing and can be expensive to implement. FIG. 2 shows a prior art arrangement for hard wiring through a mud motor 60 between downhole components (sensors, power supply, electronics, etc.) and uphole components (processor, telemetry unit, etc.). The flex shaft 66 is shown for connecting the motor output from the rotor 64 to the drive shaft 72 supported by bearings 68. The flex shaft 66 has a reduced cross-section so it can flex laterally while maintaining longitudinal and torsional rigidity to transmit rotation from the mud motor 60 to the drill bit (not shown). A central bore 67 in the flex shaft 66 provides a clear space to accommodate the conductors 46.
The flex shaft 66 is elongated and has downhole and uphole adapters 69a-b disposed thereon. The shaft 66 and adapters 69a-b each define the bore 67 so the conductors 46 used for power and/or communications can pass through them. The adapters 69a-b typically shrink or press with an interference fit to the ends of the shaft 66.
Down flowing drilling fluid from the stator 62 and rotor 64 passes in the annular space around the shaft 66 and adapters 69a-b. The shrink fitting of the adapters 69a-b to the shaft 66 creates a fluid tight seal that prevents the drilling fluid from passing into the shaft's bore 67 at the adapters 69a-b. A port 69c toward the downhole adapter 69a allows the drilling fluid to enter a central bore 73 of the drive shaft 72 so the fluid can be conveyed to the drill bit (not shown).
The flex shaft 66 has to be long enough to convert the orbital motion of the rotor 64 into purely rotational motion for the drive shaft 72 while being able to handle the required torque, stresses, and the like. Moreover, the flex shaft 66 has to be composed of a strong material having low stiffness in order to reduce bending stresses (for a given bending moment) and also to minimize the side loads placed on the surrounding radial bearings 68. For this reasons, the elongated flex shaft 66 is typically composed of titanium and can be as long as 4.5 to 5 feet. Thus, the shaft 66 can be quite expensive and complex to manufacture. Moreover, the end adaptors 69a-b shrink fit onto ends of the shaft 66 to create a fluid tight seal to keep drilling fluid out of the internal bore 67 in the shaft 66. Although the shrink fit of the adapters 69a-b avoids sealing issues, this arrangement can be expensive and complex to manufacture and assemble.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.